The invention relates to processes for photothermally inspecting a test specimen in which the test specimen is acted on by luminous radiation in an illumination region during an illumination time and thermal radiation emitted by the test specimen from a detection region is detected in a time-resolved manner, wherein properties of the test specimen are determined from the chronological course of the thermal radiation, which can be represented by means of an emissions curve that has a heating part with an increasing amplitude and a cooling part with a decreasing amplitude.
The invention also relates to devices for photothermally inspecting a test specimen, having an illumination device with which the test specimen can be acted on by luminous radiation in an illumination region during an illumination time and having a detection device with which thermal radiation emitted by the test specimen from a detection region can be detected in a time-resolved manner, and having a control and evaluation device with which properties of the test specimen can be determined from the chronological course of the thermal radiation, which can be represented by means of an emissions curve that has a heating part with an increasing amplitude and a cooling part with a decreasing amplitude.
Processes and devices of this kind have been disclosed by DE 38 20 862 A1. An illumination device acts on a test specimen with luminous radiation in an illumination region during an illumination time. A detection device is used to carry out a time-resolved detection of thermal radiation emitted by the test specimen from a detection region. When plotting a temperature in relation to time, the thermal radiation detected can be represented by an emissions curve that has a heating part with an increasing amplitude and a cooling part with a decreasing amplitude, wherein properties of the test specimen can be determined from chronological correlations between the impingement of luminous radiation on the test specimen and the course of the emissions curve. With the processes and devices of this generic type, in order to detect errors, determinations are made as to deviations of emissions measurement curves from emissions model curves that are calculated from preset parameters such as thermal conductivity, absorption coefficient, and/or emissions coefficient.
U.S. Pat. No. 4,679,946 has disclosed a process and a device for determining both a layer thickness and material parameters of a coating-like test specimen, in which the test specimen is acted on by luminous radiation and thermal radiation emitted by it is detected. Two measurement processes functioning independently of each other are used for this purpose, wherein on the one hand, thermal signals associated with a surface temperature and on the other hand, signals that exist chiefly as a function of the temperature underneath the surface are detected and processed. Through the use of a dual-segment detector, these two types of signals can be simultaneously detected independently of each other. However, this device requires two radiation sources embodied in the form of lasers as well as the specially designed detector element.
The object of the invention is to disclose processes and devices of the type mentioned the beginning with which the material properties of the test specimen can be determined in a relatively precise manner in a way that does not involve expensive equipment, even without knowledge of exact layer thicknesses.
This object is attained with the process mentioned at the beginning in an embodiment of a first type by virtue of the fact that a pulse-like illumination time is set, which is less than the quotient of the square of an estimated value of a layer thickness constituted by the distance between boundary surfaces and an estimated value for the diffusivity between the boundary surfaces and that the effusivity of the test specimen is determined from at least one measurement value disposed in the heating part or in a section of the cooling part immediately following the heating part, which has a high cooling rate that corresponds to a heating rate of the heating part.
This object is attained with the device mentioned at the beginning in an embodiment of a first type by virtue of the fact that the illumination device can set at least one illumination time that is less than the quotient of the square of an estimated value of a layer thickness constituted by the distance between boundary surfaces and an estimated value for the diffusivity between the boundary surfaces and that a calculating unit is provided, with which the effusivity of the test specimen can be determined from at least one measurement value disposed in the heating part or in a section of the cooling part immediately following the heating part, which has a high cooling rate that corresponds to a heating rate of the heating part.
With the process and device of the first type, since at least one measurement value within the very short heating part or in the partial section of the emissions curve immediately following the end of the pulse-like illumination is detected in accordance with the knowledge underlying the invention that with pulse-like illumination in this region of the emissions curve, the influence of the layer thickness on the measurement of material properties is negligible, material properties can be determined very precisely from at least one measurement value, even without knowledge of the layer thickness.
This object is also attained with the process mentioned at the beginning in an embodiment of a second type by virtue of the fact that at least two illumination times are set, which are less than the quotient of the square of an estimated value for a layer thickness of the coating and an estimated value for the diffusivity of the coating, that the illumination region and the detection region are spaced apart from each other with a diffusion spacing, that the maximal amplitudes are determined for each emissions curve, which maximal amplitudes occur for each illumination time in the detection region, with associated adjustment times, and that the diffusivity is determined from the ratio between at least two maximal amplitudes, the diffusion spacing, and the associated adjustment times.
This object is also attained with the device mentioned at the beginning in an embodiment of a second type by virtue of the fact that the illumination device can set at least two illumination times, which are less than the quotient of the square of an estimated value of a layer thickness constituted by the distance between boundary surfaces and an estimated value for the diffusivity between the boundary surfaces, that the illumination region and the detection region are spaced apart from each other with a diffusion spacing, that an emissions maximum locating unit can determine maximal amplitudes of each emissions curve which occur for each illumination time in the detection region, with associated adjustment times, and that a diffusivity calculation unit can determine a diffusivity from the ratio between two maximal amplitudes, the diffusion spacing, and the associated adjustment times.
With the process and device of the second type, since after a relatively short pulse-like heating, the propagation of thermal waves beyond the maximal amplitudes of emissions curves is detected after illuminations of different lengths, when the diffusion spacing is known, the diffusivity can be determined independently of the power density of the luminous radiation.
In suitable modifications of the process and the device of the first type, after the effusivity has been determined, the layer thickness is also determined, which can be ascertained from the long-term course of the emissions curve.
In order to increase the measurement precision of the effusivity, modifications of the process and device of the first type provide that a number of emissions measurement values are detected in the heating part of the emissions curve or in the cooling part immediately following the heating part and are compared to the standard of emissions model curves calculated from model parameters, wherein the material properties can be derived from the emissions model curve with the least deviations from the measurement values.
In suitable modifications of the process and device of the second type, two illuminations are provided, preferably with illumination times that differ by a factor of two, wherein the longer illumination time is half the quotient of the square of the estimated value for a layer thickness of a coating and the estimated value for the diffusivity of the coating. This assures that the influence of the layer thickness is negligible.
In order to explain the physical interrelationships in the process and device of the first type, some determination equations will be given below.
The effusivity is the root of the product of the thermal conductivity, the density, and the specific heat capacity. Diffusivity is understood to mean the quotient of the thermal conductivity and the specific heat capacity multiplied by the density.
For example, the non-stationary thermal conduction theory yields the following temperature course, which represents an emissions curve, at the surface of a test specimen for the heating part and the early cooling part after the end of illumination with a short rectangular pulse:                               T          ⁡                      (                                          z                =                0                            ,              t                        )                          =                                            2              ⁢                              F                0                                                    π                                ·                      1            E                    ·                      {                                                                                t                                                                                                              for                      ⁢                                              xe2x80x83                                            ⁢                      0                                        ≤                    t                    ≤                                          t                      p                                                                                                                                        (                                                                  t                                            -                                                                        t                          -                                                      t                            p                                                                                                                )                                                                                                              for                      ⁢                                              xe2x80x83                                            ⁢                      t                                        ≥                                          t                      p                                                                                                                              (        1        )            
wherein T(z=0, t) is the temperature at the surface of the test specimen, F0 stands for the introduced power density in W/cm2, E stands for the effusivity, tP represents the illumination time, and t represents the time since the beginning of the illumination. With the detection of one or a number of emissions measurement values at known times ti which lie in the heating part of the emissions curve or in the early cooling part immediately following the heating part, as well as the associated temperatures Ti, if the power density F0 is known, the above equation can be used to determine the effusivity, independent of the layer thickness.
With the process and device of the second type, the determination of the diffusivity is based on the following equation:                                           T            1                                T            2                          =                              erfc            ⁢                          xe2x80x83                        ⁢                          r                                                4                  ⁢                                      xe2x80x83                                    ⁢                  α                  ⁢                                      xe2x80x83                                    ⁢                                      t                    1                                                                                            erfc            ⁢                          xe2x80x83                        ⁢                          r                                                4                  ⁢                                      xe2x80x83                                    ⁢                  α                  ⁢                                      xe2x80x83                                    ⁢                                      t                    2                                                                                                          (        2        )            
wherein T1, T2 stand for the temperatures occurring after illumination with illumination times t1, t2 in a diffusion spacing r from the illumination region, xcex1 stands for the diffusivity, and xe2x80x9cerfcxe2x80x9d stands for the so-called complementary Gaussian error function in the form:                               erfc          ⁡                      (            x            )                          =                  1          -                                    2                              π                                      ⁢                                          ∫                0                x                            ⁢                                                e                                      -                                          y                      2                                                                      ⁢                                  ⅆ                  y                                                                                        (        3        )            
Through variation of the diffusivity xcex1, the equation (2), which has one value due to the temperature measurement, the knowledge of the diffusion spacing, and the knowledge of the illumination times, can be fulfilled with a diffusivity xcex1.